1. Field of Invention
The present invention relates to system and method for processing combustion exhaust gas containing soot particles and NOx.
2. Description of Related Technology
Exhaust gas emitted from a diesel engine includes air pollutants, such as carbon monoxide (CO), unburned hydrocarbon (HC), nitrogen oxide (NOx), particulate matters containing soluble organic fractions (SOF), and the like. The permissible amount of exhaust gas emitted from a diesel engine is generally regulated by law and often determined by means of the management and design of engine and the post processing of exhaust gas.
In a general diesel engine, HC and CO are purified to a predetermined level by means of Pt-based diesel oxidation catalyst (DOC), NOx emission is controlled by means of an exhaust gas recirculation (EGR) system of a diesel engine, and particulate matters (PM) are collected by means of a diesel particulate filter (DPF) such that the amount of particulate matters to be discharged will be controlled within a predetermined range. However, such a technique is performed by causing particulate matters collected in the diesel particulate filter to be burnt at a high temperature using an additional device such as heater or burner or by causing the particulate matters to be oxidized using catalyst coated on an inner surface of the diesel particulate filter to regenerate the filter. Therefore, its utility is greatly reduced.
As another technology, the Johnson Matthey company has proposed such a technique that a diesel particulate filter is regenerated by causing NO in exhaust gas to be oxidized into NO2 having superior oxidation activity upstream of the diesel particulate filter and causing particulate matters collected in the diesel particulate filter to be oxidized using the oxidized NO2 as an oxidizing agent. Such a technique is disclosed in EP-A-0341832 and U.S. Pat. No. 4,902,487 and is well known in the art as a trademark CRTTM.
According to the aforementioned technique, however, platinum (Pt) susceptible to the poisoning due to sulfur existing abundantly in diesel exhaust gas is used as an oxidizing agent for causing NO to be oxidized into NO2. Therefore, there is a limitation in that only fuel such as ultra low sulfur diesel (ULSD) should be used. Further, since about 3 to 8% of NOx can be simply reduced from the exhaust gas, there is a need for an additional technique for reducing nitrogen oxide.
In this connection, the Johnson Matthey Company has developed a four-way post-processing system called a SCRTTM system in which selective catalytic reduction (SCR) catalyst is applied to a diesel particulate filter. This system is known as a system capable of reducing particulate matters and nitrogen oxide by about 75 to 90% as well as removing HC and CO. However, this system requires post-processed volume of the exhaust gas 13.6 times as large as the engine displacement. Therefore, it is tentatively applied to a large diesel engine but came to light that its utility is greatly lowered. Further, the poisoning problem due to sulfur still remains.
Furthermore, Toyota Motor Corporation has developed a diesel particulate NOx reduction (DPNR) system by which both particulate matters and nitrogen oxide can be reduced by causing NOx adsorber catalyst to be coated onto a general diesel particulate filter (DPF) and platinum oxidation catalyst to be placed onto the diesel particulate filter. This system is disclosed in Japanese Patent Laid-Open Publication No. (Hei) 6-159037 and the like.
The DPNR technique will be described with reference to FIG. 1. In FIG. 1(a) showing a lean burn condition of a diesel engine, O2 and NO react with each other near oxidation catalyst, i.e. Pt, and active oxygen O* and NO2 are produced. Further, NO2 exists in adsorber catalyst in the form of its salt. Then, particulate matters (PM) are oxidized by means of O* so produced and O2 in the exhaust gas, and the oxidized PM again reacts with O2 in the exhaust gas to be oxidized into CO2.
In addition, in FIG. 1(b) showing a rich burn condition of a diesel engine, NOx, which has been adsorbed to the adsorber catalyst in the form of its salt by means of hot temperature and instantaneous rich exhaust conditions, is converted into NO and O* which in turn are discharged. Further, the discharged NO and O* react with HC and CO through oxidation catalytic reaction and are converted into CO2, H2O and N2. Furthermore, even under the rich burn condition where there is shortage of oxygen, the particulate matters (PM) reacts with O* emitted from the adsorber catalyst such that it can be oxidized into CO2.
However, the DPNR system of Toyota Motor Corporation, in which a continuous regeneration approach is employed, still has the following technical limitations. First, since platinum is used as oxidation catalyst, reduction in performance resulting from the poisoning due to sulfur existing abundantly in diesel exhaust gas cannot be avoided. Second, since the adsorbed nitrogen oxide can be purified only if a rich burn condition should be periodically provided, high fuel efficiency that is one of most significant advantages of a diesel engine is lowered. Further, there are problems in that costs are increased because an additional fuel injection system should be installed to an upper end of the DPNR system in order to provide a periodic rich burn condition, and that a stable operation cannot be performed because the post injection should be made in the fuel injection system of the diesel engine.
In addition, since there is a limit to the amount of NOx adsorbed by the adsorber catalyst under the lean burn condition of a diesel engine, the diesel engine needs to be periodically operated under a rich burn condition or at a stoichiometric air-fuel ratio where there is shortage of oxygen. This means that the aforementioned conventional technique is not suitable for a diesel car engine that usually operates in a lean burn condition without the occurrence of a periodic high-load condition in view of its combustion characteristics.
Furthermore, the conventional technique has the following problems. That is, high activation temperature and resultant high energy are required to allow platinum constituting the oxidation catalyst to have sufficient oxidizing power, and the platinum oxidation catalyst cannot provide any functions of purifying the exhaust gas until it reaches the activation temperature.
Moreover, according to the conventional technique, since HC and CO are rapidly oxidized by means of the platinum oxidation catalyst under the rich burn condition where NOx adsorber catalyst can be regenerated, the reduction of NOx can be prevented. To avoid this, the conventional technique adopts the post injection method in which greater amount of fuel is injected in consideration of the amount of HC and CO under the rich burn condition. However, this results in waste of diesel fuel.